Insect Physiology Laboratory, Section of Entomology, Department of Zoology, Aligarh Muslim University, Aligarh-202 002, Uttar Pradesh (India).
In the newly moulted fifth instar larvae of Utetheisa pulchella Linn., invertase activity was significant in the homogenates of whole midgut (midgut tissue + midgut contents) suggesting the carry-over of enzymes in the contents of midgut from previous instar. When the food was continuously available to the caterpillars following moulting, the invertase activity of the midgut increased and peak of maximal activity in the tissue occurred in 24 h. Also, the enzyme activity was high in the first half of the instar which may also be regarded as period of high metabolic activity. When the larvae were unfed following moulting, some invertase was produced which also showed a peak of maximal activity in the midgut tissue in 24 h. This trend is similar to that of the fed category. In this case, the enzyme production may be under the influence of some endogenous factor responsible for the initial stimulus for the secretion of digestive enzymes. The same situation existed in the fourth instar larvae as well. Thus, the secretory behaviour of digestive enzyme is almost alike in different larval instars of U. pulchella Moreover, in the fed or unfed larvae of different ages, the tissue enzyme activity was significantly less than that of whole midgut. This indicated that in the tissues, the activity of invertase represented the index of enzyme production. The enzyme was washed off concurrently from the tissue and accumulated in the midgut lumen. Evidently, synthesis and discharge of digestive enzymes seem to be integrated processes.
